BSP-020: Water Management Details for Residential Buildings

Probably the most consistent form of building enclosure failure in most climate zones is through water penetration from window installations.  Windows are a key part to almost every building, as they improve the indoor environmental quality through daylighting, views, and in some cases ventilation.  Yet they are also one of the most difficult components of the building enclosure to incorporate into the water management system.

Leaks from windows have existed as long as windows have existed.  Water not only leaked through the windows themselves, but also around the windows at the interface between the rough opening and the window frame.  In many older solid masonry buildings this was not a problem due to the capacity of the construction to absorb moisture and dry out under more favorable conditions.  As the construction industry changed, materials and assemblies began to be used that had less of a capacity to manage moisture.  The absorption capacity and the drying potential of assemblies were significantly reduced.  This led to serious building enclosure failures related to window leakage.

Figure 19: Water intrusion through the window frame

Installation Concerns

Risks associated with window leaks are affected by various characteristics from the climate in which the window is installed, to the materials used, to the skill and attitude of the person doing the installation.  Understanding these factors including the limitations and critical aspects of the installation is important in deciding the appropriate installation technique to use.  

In general there are two basic window installation strategies: a barrier approach or a drained approach.  

The barrier approach works under the preconception that the installation will keep all the water out and is the approach used by much of the industry.  In this approach, the window is sealed on all four sides in an attempt to create a perfect barrier against water infiltration.  Experience has shown that most window installations do not have the ability to perform as a perfect barrier for the life of the window installation.  Due to this, barrier systems, while a form of water management, are not generally recommended and should only be considered for use in certain assemblies such as mass wall assemblies.  

Drained systems work under the preconception that some water will leak through the window at some point in time, and provisions are made to direct the water back out to the exterior.  This approach has been shown to be an effective method of water management, though care must still be taken in the design and application of the system.

Nailing Flange

Most residential windows on the market are designed with a nailing flange to help with the installation of the window.  The flange is intended to make consistently lining up the window in the same location in the rough opening easier.  The flange, however, is often not structural and proper shimming and blocking of the window is required.  This nailing flange has some advantages and some disadvantages associated with it.  While it does make the installation easier, the water management strategies must be carefully considered.   

The nailing flange provided a good opportunity to be used as a part of the drainage plane of the wall assembly for both barrier as well as drained window installations.  The flat flange installed (for the most part) at the plane of the exterior sheathing seemed to be a perfect match to integrate the window into the moisture management system of the home.  It does work very effectively, as long as the flange is continuous around the entire window perimeter and integrity of the flange is maintained.  

The nailing flange is usually a thin PVC or metal fin that is part of the frame extrusion and generally not very strong.  These flanges can be damaged, bent or broken during the delivery or installation of the window system into the wall assembly, or may not be continuous around the window depending on the assembly at the corner joints of the frame.  Damaged and broken nailing fins can lead to fundamental failures in the integrity of the drainage plane of the wall assembly as membrane tapes and building papers/housewraps no longer have adequate area of material to adhere to or cover over the flange.

As long as the nailing flange is continuous and undamaged it can be effectively integrated into the moisture management of the wall assembly.  Self adhered membranes are often used to integrate the nailing flange into the drainage plane of the water management system of the wall assembly.  Self adhered membranes used to seal to the nailing flange must be installed, as with other water management concepts, in a shingle lap fashion to prevent reverse laps that can create paths for water intrusion.  While this concept is generally understood, repetitive installations and requirements to complete the installations in a timely manner, has led to poor installations and problems with installation.

Often, due to the different trades on the site, or the sequencing of materials being delivered to the site, the windows may be installed before or after the installation of the drainage plane material.  In either case, provisions must be made to allow the window to be integrated into the drainage plane of the assembly.  It is important to ensure that the building paper or housewrap laps over the flange and self adhered membrane at the head of the window.  If the building paper/housewrap is installed prior to the window, then a flap of paper at the head should be turned up during the installation of the window, then the flap can be dropped back down over the top of the flange.  Using self adhered membrane flashings and other tapes to seal the nailing flange at the head of the window, while reducing the risk of water intrusion, still create a reverse lap and must remain completely sealed for the service life of the installation in order to be effective.  

Pan Flashing

The installation of a pan flashing at the sill of the rough opening is critical to the design of drained window installations.  They are used to protect the framing of the rough opening from moisture damage and to help direct any water back out to the exterior.  These pan flashings can be created out of numerous different types of materials, from preformed plastics, membrane flashings, and metal.  

Ideally, the pan flashing would be sloped to the exterior.  This can be achieved through the use of a section of board siding laid flat under the pan flashing.  In cases where positive slope to drain is not practical, then a back dam should be installed to prevent water from draining back to the interior.

The condition of the rough opening can affect the performance of the drainage of the pan flashing.  The exterior sheathing must not extend higher than the sill framing.  In this situation, even with a perfectly installed pan flashing, the drainage of the system will be compromised.

Figure 25: Drainage blocked due to exterior sheathing above the sill framing

The geometry of the sill with a back dam creates a situation where several planes of construction come together in a small area.  With membrane flashings this can lead to installation problems due to the need to use a two dimensional product in a 3 dimensional application.  The membranes must be folded, cut, and layered in such a way as to protect the surrounding construction.  The time and care that this takes is not always done.  Problems that can arise are tenting of the membrane in the corners, holes, and “fishmouths”.  

Tenting of the membranes occurs when the membrane pieces are not seated properly into the corners of the sill rough opening.  The result is a curved fillet, placing the flashing in tension with a void beneath.  Because of this tearing of the membrane can occur during the installation of the pan flashing, or the sharp corners of the window frame can cut or damage the reinforcing of the membrane leading to failures in the integrity of the pan flashing.

Figure 27: Torn membrane pan flashing

Figure 28: Window corner cutting into improperly seated pan flashing

Rushed installations also lead to problems with poor membrane installations and “fishmouths”.  “Fishmouths” act as a funnel to direct water into the construction.

As a means to correct some of these concerns, other products available on the market such as moldable membrane flashings and preformed plastics can be used for the installation of the pan flashing.  Moldable membrane flashings allow for a single piece of membrane to be installed at the corner of the sill as the membrane can stretch to fan out and curve around corners.  

Preformed plastic pan flashings have also been developed to address this problem of time consuming and finicky work of creating pan flashings with membranes.  The preformed plastic sections usually have integrated backdams as well.  With these products the jambs and sill joint between the two pieces must still be sealed with membrane to prevent water intrusion below the pan flashing.

Figure 30: Moldable membrane and preformed plastic pan flashing

Furred Out Windows

Many architectural designs attempt to push the window frame further to the exterior of the wall assembly for aesthetic reasons.  While it is possible to design the water management system to function adequately with this type of window installation, the difficulties associated with trying to move the window further to the exterior generally do not make this a recommended approach.  

The furring strips move the window out of the drainage plane of the wall assembly.  This adds more three dimensional complexity to the window, flashing, and housewrap installation details.  Clearance requirements between adjoining assemblies can also become an issue.  Most sheet good products become difficult to install in locations where there are multiple changes in plane in a relatively small location, often leading to poor installations and discontinuities in the drainage plane.